ERNST G. HUF 



207 



where waler How is in tlie opposite direction. Now, if frog skin separates two 

 solutions of different osmotic pressure, for example frog Ringer's and o.i frog 

 Ringer's, one will notice that outward osmosis is quite a bit faster than inward 

 osmosis (fig. i). 



The phenomenon of 'one-way osmosis' is very easy to demonstrate. An 

 argument, however, arose as to whether one should say, then, that the water 

 permeability of the skin is smaller in the inward than in the outward direction. 

 In an often-quoted paper by v. Hevesy, Hofer and Krogh (12), it was shown 

 that the permeability of the skin for heavy water (D2O) in Ringer's was the 

 same, no matter whether it penetrated from the outside or from the inside of 

 the skin. It was felt (13) that the experiments of v. Hevesy et al. settled all 

 arguments about 'one-way water permeability' of frog skin, simply by denying 

 its existence. Obviously, this was not justitied. The same authors had also found 



Fig. I. One-way osmosis across 

 isolated frog skin in a differential 

 osmometer. Three consecutive ex- 

 periments of one hour duration 

 each, using the same piece of skin 

 from the abdominal wall of Rana 

 esculenta. Starting off with outward 

 osmosis, i.e. with Ringer's on the 

 epithelial side and 10 Ringer's on 

 the corium side. In inward osmosis 

 the osmotic gradient was reversed 

 using again Ringer's and ro Ring- 

 er's. 21°. i — + o and o —* i indicate 

 the direction of osmosis from the in- 

 side — > outside and the outside — > 

 inside, respectively (19). 



0,5 



1,0 0,5 1,0 0,5 Hrs. 1,0 



that on the basis of equally effective molar concentration differences, osmotic 

 flow of ordinary water is, on the average, four times greater than the rate of 

 diffusion of D2O. Similar observations on frog skin were recently made by 

 Capraro and Bernini (i). This leaves one with the impression that the structure 

 of the skin enters, in different ways, into the processes of osmosis and water 

 diffusion. In dealing with the movement of water across the skin (and other 

 structures as well) one must clearly distinguish between 'osmo-permeability' 

 and 'diffusion permeability' (23). If it is correct that the rate of diffusion of 

 heavy water across skin is the same in both directions, one must conclude that 

 'one-way osmosis' cannot be understood from the standpoint of diffusion. No 

 one deserves more credit for establishing clear terms in the field of permeability 

 and permeability for water, in particular, than M. H. Jacobs (30, 31). He 

 pointed out that an assumption often made in the past, "that a concentration 

 gradient of solute determines a concentration gradient of water of equal 



